Did you get the email headers from a customer that is reporting this? You can analyze the headers to see who is flagging you. Are your emails generated on DO and you relay through another service (SES), but you are not obfuscating the DO header that your MTA injects? [1]
[1] - https://major.io/2013/04/14/remove-sensitive-information-fro...
I would even go as far as to suggest that both they and their customers should have aliases on all domains for spam@, uce@, postmaster@, abuse@, malware@ and security@ that route to a distribution list or mailbox that the domain holder responds to that same day. Back in the day, that was a requirement to varying degrees to sign up for any of the big email campaign providers.
Every domain holder should also have a proper DNS dmarc rule with a reporting email address listed. Example using ycombinator.com [1]
[1] dig +short _dmarc.ycombinator.com txt
"v=DMARC1; p=none; sp=none; rua=mailto:dmarc-reports@ycombinator.com,mailto:re+gewxcbuqfmh@inbound.dmarcdigests.com; aspf=r; pct=100"